1. Field of the Invention
The present invention relates in general to a piercing die and a piercing method for piercing a metallic workpiece sheet, and more particularly, to such piercing die and method using an edge-chamfered punch that permits a piercing operation without burrs on the obtained product.
2. Discussion of the Prior Art
In the art of manufacture of automotive components such as a door panel and a hood panel by press-forming techniques, there has been used a piercing die which has a punch block movable in a piercing direction, and a die block disposed below the punch block. The punch block carries a punch, while the die block supports a die having a die hole into which the punch is moved with a suitable clearance, to perform a piercing operation on a workpiece in the form of a sheet or strip.
The piercing operation is performed such that the punch is lowered onto the workpiece sheet placed on the die block, whereby an appropriate portion of the workpiece sheet is subjected to a shearing stress due to a shearing action caused by the edges of the punch and die, when the punch penetrates into the sheet and the sheet bulges into the die hole. As a result, cracking occurs on the workpiece sheet, starting from at least one of the opposite surfaces of the sheet which contact the punch and die, respectively. As the piercing movement of the punch relative to the die progresses, a portion of the sheet corresponding to the periphery of the punch is removed. Thus, the workpiece sheet is pierced, and a pierced product is obtained.
However, the cracking of the sheet may occur either at the edge of the hole to be formed by piercing, i.e., near the edge of the die hole, or alternatively at the edge of the portion of the sheet to be removed, i.e., near the edge of the punch. Namely, the portion at which rupture occurs may be on the side of the punch or die. If the rupture occurs on the side of the punch, the obtained pierced product has burrs along the edge of the hole formed by piercing.
To assure burr-free piercing or blanking operation, it is known to round or chamfer the edge of one of the punch and die which corresponds to the product to be obtained by the piercing or blanking operation, as disclosed in laid-open Publication No. 1-293922 of unexamined Japanese Patent Application. For example, the edge of the punch is chamfered while the edge of the die remains sharp, so that the edge of the hole formed through the pierced product is free of burrs.
More specifically described by reference to FIGS. 8 and 9, a punch 51 has the lower working end whose outer periphery is chamfered, while a die 52 has a die hole whose edge 55 remains sharp. In this arrangement, a shearing stress is concentrated on a portion of a metallic workpiece sheet 56 near the sharp edge of the die 52, whereby cracking tends to take place at that portion of the sheet 56 near the die edge 55, starting from the lower surface of the workpiece sheet 56, as shown in FIG. 9. Consequently, the sheet 56 tends to rupture at the cracked portion near the sharp edge of the die 52, so that the pierced product having a hole is less likely to have burrs along the edge of the hole, while the burrs are likely to be formed along the edge of the removed portion of the sheet 56, namely, left on the scrap. Reference numeral 59 denotes a stripper for holding the workpiece sheet 56 against the die 52.
A graph in FIG. 10 indicates reduction in the height of the burrs left on the product obtained by the punch 51 and die 52 of FIGS. 8 and 9, as compared with that obtained by the conventional ordinary punch and die. The height of the burrs varies with the percent (%) of the clearance between the punch and die with respect to the thickness of the workpiece sheet 56.
The workpiece sheet 56 is made of a metallic material such as steel, aluminum, copper, zinc, titanium or alloys thereof, for example, and has a thickness between 0.2 mm and 6 mm. The amount of chamfer formed along the periphery of the lower end of the punch 52 should be at least 0.2 mm, and preferably smaller than a value close to the thickness of the workpiece sheet 56. While the edge 5 of the die 52 should be sharp, the die edge 55 may be rounded, but the radius of the rounded edge 55 should not exceed 0.15 mm.
However, the present applicants found a drawback on the known arrangement of FIGS. 8 and 9, when the punch and die 51, 52 are used to effect a piercing operation in which a non-circular hole such as a rectangle or ellipse having portions with different curvatures is formed through the workpiece sheet 56, by using the punch and die 51, 52 whose configurations correspond to that of the hole to be formed. That is, it was found that the pierced product had a considerable amount of burrs at portions of the formed hole where curvatures are relatively large.
While the mechanism for the drawback is not clear to the applicants, it is presumed that a portion of the sheet 56 which corresponds to a portion of the punch 51 (die 52) having a relatively small curvature is subjected to reduction in its thickness, due to pressure of the punch 51 acting on that portion squeezed between the punch 51 and the die 52 as a result of the lowering movement of the punch 51. When the thickness of that squeezed portion is eventually reduced below a given value, the sheet 56 undergoes cracking and then rupture at a part thereof near the sharp die edge 55. On the other hand, a portion of the sheet 56 corresponding to a portion of the punch 51 which has a relatively large curvature, for example, a portion of the sheet 56 corresponding to a corner of a rectangular profile of the punch 51 (die 52), tends to have a larger thickness than the portion corresponding to the portion of the punch 51 having the relatively small curvature. This larger thickness seems to arise from a flow of the material toward the large-curvature portions of the punch 51 and die 52, which flow occurs in the direction perpendicular to the piercing direction. As a result, the portions of the material of the sheet 56 near the large-curvature portions (e.g., corner portions) of the die edge 55 are less likely to have reduced thickness, than the other portions near the small-curvature portions of the die edge 55. In other words, the reduction of the material thickness at the portions corresponding to the large-curvature portions of the die edge 55 is delayed with respect to that at the portions corresponding to the small-curvature portions of the die edge 55, whereby there may arise cracking and then rupture of the material at the portions of the material near the large-curvature portions of the punch 51, before cracking of the portions of the material near the large-curvature portions of the die edge 55, due to a tensile stress at the former portions exceeding a threshold value. Namely, the portions of the material near the large-curvature portions of the die edge 55 would not crack before rupturing, but the portions of the material near the large-curvature portions of the punch 51 first cracks and then ruptures. FIG. 11 shows cracking of the sheet 56 relatively near the punch 51, and near peripheral portions of the punch and die 51, 52 whose curvatures are larger than the other portions. While the portion of the material 56 corresponding to the punch 51 is eventually removed by a further piercing movement of the punch 51, the ruptured portions near the large-curvature portions of the punch 51 are left as burrs around the hole formed through the obtained pierced product, which burrs protrude beyond the back surface of the sheet 56 in the piercing direction.
Sometimes, a piercing operation is effected, with the workpiece sheet in an inclined attitude, while being placed on a die whose upper surface is inclined with respect to the horizontal direction perpendicular to the movement of an upper punch relative to the die. For holding the height of burrs left on the pierced product within a permissible or tolerable range (e.g., less than 0.1 mm), the percent of the clearance between the punch and the die with respect to the thickness of the workpiece sheet should be held within a relatively narrow range, as indicated in the graph of FIG. 12.
Further, the piercing operation on an inclined workpiece sheet as described above usually suffers from an interference of the punch with the die, due to a sliding force acting on the working end face of the punch which contacts the inclined surface of the workpiece sheet. If the sliding force exceeds a static friction force between the end face of the punch and the inclined surface of the sheet, the punch is forced to slide toward the lower portion of the edge of the die, whereby the outer periphery of the punch may eventually contact and thereby damage the die.
To avoid such interference between the punch and the die, the above-identified Japanese laid-open Publication No. 1-293922 proposes the application of chamfering of a punch to a piercing operation on an inclined workpiece sheet as described above. Namely, a punch 61 has an inclined lower working end face, while a die 62 has an inclined top surface parallel to the inclined punch end face, as shown in FIG. 13. The punch 61 has a chamfer 61a along the outer peripheral edge at the working end face, while the die 62 has a sharp edge 62a. The chamfered edge 61a of the punch 61 assures cracking of a workpiece sheet, at a portion near the sharp die edge 62a, so as to minimize the amount of burrs left on the obtained product, as described above. This arrangement is effective to reduce the burrs, without reducing the clearance between the punch and die 61, 62, and is therefore free from damage of the punch and die 61, 62 due to an interference therebetween.
However, the above arrangement also suffers from a drawback, which seems to occur due to a difference in angle formed by the inclined working end face of the punch 61 and the piercing direction of the punch, between the diametrically opposed portions of the outer periphery at the working end face of the punch. More specifically, the outer peripheral edge of the punch 61 has, at its lower and upper peripheral portions, an acute portion and an obtuse portion which form an acute and an obtuse angle with respect to the piercing direction, respectively. For easy understanding, the portions of the chamfer 61a formed at the acute and obtuse portions will be referred to as "acute side 68" and "obtuse side 69", respectively, which are indicated in FIG. 13.
A piercing operation effected by the punch 61 and the die 62 is illustrated in FIG. 14, in which reference numerals 63 and 64 denote a stripper and a workpiece sheet. It appears that a portion of the workpiece sheet 64 near the acute side 68 of the chamfer 61a of the punch 61 tends to easily crack and then rupture at a portion near the sharp die edge 62a, while a portion of the workpiece sheet 64 near the obtuse side 69 of the chamfer 61a tends to rupture at the portion near the sharp die edge 62a, before the same portion undergoes a sufficient degree of cracking due to shearing and tensile stresses. Consequently, it is considered that burrs are likely to be left at the portion of the sheet 64 near the obtuse side 69, due to the rupturing without the prior sufficient cracking starting from the surface on the side of the sharp edge 62a, which causes the ruptured portion to protrude from the back surface of the sheet 64 in the piercing direction. Thus, the pierced product tends to have burrs at the portion corresponding to the obtuse side 69 of the chamfer 61a of the punch 61.